This invention relates to a transparent substrate for a mask blank and a mask blank and, more specifically, to a transparent substrate for a mask blank and a mask blank which are capable of preventing deviation from specification with respect to an optical characteristic of the mask blank by assuring an optical characteristic of the transparent substrate or a thin film.
Proposal has been made of a transparent substrate or a mask blank which is characterized by a specially designed shape of a corner portion of the transparent substrate or a specially designed shape of a peripheral portion of a thin film formed on the transparent substrate (for example, see Japanese Examined Utility Model Application Publication (JP-Y) No. 63-8900 (patent document 1), Japanese Unexamined Patent Application Publication (JP-A) No. 2000-356849 (patent document 2), Japanese Unexamined Utility Model Application Publication (JP-U) No. 60-39047 (patent document 3).
The patent document 1 discloses a transparent substrate having a substrate mark formed on the corner portion in order to discriminate a material of the transparent substrate.
The patent document 2 discloses forming a substrate mark having an asymmetrical shape with respect to a diagonal line in order to discriminate many types of transparent substrates.
The patent document 3 discloses uniformly forming a light shielding film (an opaque film) on a transparent substrate except for a peripheral portion and a side surface. With this structure, during use of the mask blank, the light shielding film is not peeled from the peripheral portion and the side surface. It is therefore possible to prevent a pattern defect caused by dust generation.
However, each of the substrate marks disclosed in the patent documents 1 and 2 has only a function for discriminating the material of the transparent substrate while an uncoated portion without the light shielding film disclosed in the patent document 3 has only a function for preventing the dust generation from the mask blank.
On the other hand, recently, following miniaturization of a semiconductor device, the wavelength of an exposure light source to be used has been progressively shortened. Specifically, the exposure wavelength has reached 200 nm or less. For example, as such an exposure light source, use is made of an ArF excimer laser (wavelength of 193 nm), an F2 excimer laser (wavelength of 157 nm) or the like. Rapid development is made of a light shielding film for shielding light for these exposure light wavelengths or a phase shift film for shifting a phase of the light. As those films, a wide variety of film materials have been proposed (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2002-162727 (patent document 4) and Japanese Unexamined Patent Application Publication (JP-A) No. 2003-280168 (patent document 5)).
Further, several proposals are made of a manufacturing method capable of suppressing variation in optical characteristic (for example, transmittance or phase difference), which is expected to cause problems upon forming these films (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2002-90978 (patent documents 6)). Thus, at present, the variation of the optical characteristic of the films has been considerably suppressed.
However, when the optical characteristics (transmittance, reflectance or the like) of manufactured mask blanks were measured, those mask blanks which do not satisfy the specification with respect to the variation of the optical characteristics encounter problems at a certain ratio.
The present inventor has investigated the cause of the above-mentioned problem from various viewpoints. As a consequence, it has been found out that the variation of the transmittance is caused by absorption of the transparent substrate itself for the exposure light, which conventionally caused no problem.
Recently, a synthetic quartz glass is used as a substrate material of a mask blank for use with, as the exposure light source, the ArF excimer laser which has been rapidly developed at present. The synthetic quartz glass is also used as a substrate material of a mask blank for use with, as the exposure light source, the KrF excimer laser which is practically used at present. The exposure wavelength of the KrF excimer is 248 nm. Therefore, even if the synthetic quartz glass has production variation, the transmittance (the transmittance in a plate thickness direction) is 88% or higher (wavelength λ: 240 nm) for a 6025 size (thickness of 6.35 mm). Thus, no problems are caused.
However, if the wavelength of the exposure light source becomes short like in the ArF excimer laser (wavelength of 193 nm), the transmittance (the transmittance in the plate thickness direction) is sometimes 88% or lower for the 6025 size (thickness of 6.35 mm) because of the absorption of the substrate itself for the exposure light due to the production variation or the like of the synthetic quartz glass. Such reduction of the transmittance becomes remarkable in case where the exposure light has the wavelength of 200 nm or less (in particular, 140 nm to 200 nm).
In the present status, the production variation upon forming the thin film is not completely eliminated. As mentioned above, those mask blanks, which do not satisfy the specification with respect to the variation of the optical characteristic, are considered to deviate from the specification by a synergistic effect of the variation of the transmittance of the substrate material and the variation of the optical characteristic of the thin film.